1. Field of the Invention
This invention relates to novel siloxane compositions and to the use of such compositions in the fabrication of fire- and impact-resistant safety glass laminates. More particularly, the invention is concerned with the provision of optically clear silicone interlayers that can be applied between two or multiple sheets of glass or other brittle materials by casting-in-place and curing them at room temperature or elevated temperatures. Furthermore, the invention relates to glass laminates made with the cast-in-place silicone interlayers and their use as barriers to fire or impacting bodies.
2. Background Information
Laminated structures such as windows and windshields comprising sheets of glass and/or plastic having therebetween an interlayer of organic or organosilicon material are known. For example G.B. Patent Specification 783 867 discloses organopolysiloxane compositions which are convertible to transparent elastomers and which are useful as interlayers in the preparation of safety glass. French Patent No. 2 213 162 discloses sound insulating windows comprising two spaced sheets of glass in which the space is filled with a silicone elastomer.
Impact resistant, optically clear, laminated safety glass comprising sheets of glass and/or plastic having therebetween an interlayer of organic or organosilicon material are known for use principally as windshields in vehicles or aircraft and as windows in commercial and private buildings. The safety glass is manufactured by either autoclaving the glass and/or plastic panes with performed (calendered) sheets of elastomeric or thermoplastic interlayer or by pouring corbel compositions in the space between the glass panes and curing them in place (cast-in-place). Polyacetals, especially polyvinylbutyral, and to some extent silicones, based on high molecular weight gums and highly reinforcing silicas, are used for the autoclaving process. Polyacrylates, such as polymethacrylate, are typically used in the cast-in-place process, which is of particular interest because of the low processing equipment costs associated with it.
The performance of such glass laminates in fire and impact resistance tests not only depends on the material used as interlayer, but also on the type of glass. Regular silicate glass (float glass), wired silicate glass, heat strengthened (annealed or tempered) silicate glass, heat strengthened borosilicate glass, glass ceramic, or visually clear thermoplastics (polyacrylate, polycarbonate) can be used for manufacture of the glass laminate. Regular silicate glass is of particular interest because of its low material cost and is the preferred material of choice for manufacture of laminated glass targeted at the lower safety requirement classes in the building industry.
When safety glass manufactured from regular silicate glass sheets is exposed to fire, the glass directly exposed to the fire cracks almost immediately, exposing the interlayer to further attack from the fire. When such safety glass has been manufactured with thermoplastic organic interlayer, the heat radiated from the fire will cause the interlayer to melt and flow through the cracks in the glass. The molten thermoplastic interlayer will then drip to the base of the assembly, where it is consumed by the fire, typically within a few minutes. Continued fire exposure results in pieces of glass falling away and eventual disintegration of the laminated glass structure. As further glass panes crack, the process repeats itself, until the complete glass structure collapses, and fire and smoke are free to penetrate the opening. Safety glasses manufactured by laminating two sheets of regular silicate glass with a thermoplastic organic interlayer, whether performed or cast-in-place, do not pass a 30 minute fire endurance test; the minimum required to achieve a fire resistance rating.
Attempts to overcome the poor fire performance of glass laminates made with organic thermoplastic interlayers have involved the use of wired glass, where a wire mesh is employed to provide strength to the glass. However, the visually apparent wire mesh tends to detract from the aesthetics of the window. Other attempts focused on improving the heat stability and/or the fire resistance of the thermoplastic interlayer. For example Gomez in U.S. Pat. No. 4,681,810, issued Jul. 21, 1987, and in U.S. Pat. No. 4,704,418, issued Nov. 3, 1987; and Amendola et al in U.S. Pat. No. 4,704,304, issued Nov. 3, 1987, disclose the addition of a plasticizer blend of a char-forming component, such as an organic phosphate, and an oxygen sequestering agent, such as an organic phosphite, to a polyvinyl butyral (PVB) composition. These attempts, however, typically decrease the UV resistance of the interlayer, causing it to discolorate over time.
Laminated glass structures having fire resistant properties have been developed wherein the interlayer is formed from an organic resin. Also, there is disclosed ill French Patent No. 2 394 394 a fire resistant window comprising at least two sheets of glass having therebetween a sheet of silicone elastomer, characterized ill that the sheet of glass facing tile fire is not fire resistant and the other sheet of glass is fire resistant.
In general, thermoplastic organic interlayers suffer from poor weather-resistance, especially poor hydrolytical stability, which requires special glazing measures.
Other safety glasses are manufactured by casting sols of mainly inorganic nature between silicate glass panes. As the sol gels, it provides additional stability to the laminate structure. Since these interlayer gels contain water, they foam in place when exposed to the radiated heat of a fire. As the water evaporates from such interlayers, they discolor and, thus, shield the outer glass pane from heat radiation. These special interlayer gels are very costly and in normal, non-fire use have more color (yellowness) and further reduced weatherability (resistance to UV) than do conventional organic thermoplastic interlayers.
Impact resistance of laminates made from brittle materials is generally achieved through the use of though interlayers. As the body impacts on the laminated structure, its kinetic energy is transformed into shock waves that travel through the glass plates and the interlayer. For low impact energies, the glass laminate deflects only elastically, the impacting body does not cause crack formation in the impacted glass plate. The same phenomenon occurs in the initial state of a high energy impact: The glass laminate deflects elastically up to a certain maximum energy stored, then a crack is formed in the impacted glass plate, which propagates in the further deflecting plate. As the glass laminate continues to deflect, cracks are formed in the second glass plate, and potentially in further glass plates even farther remote to the impact. Since glass is a brittle material, the load rapidly approaches zero within a very small total displacement, once a crack has initiated in a glass plate. The fracturing of the glass plate, therefore, contributes only a limited amount to the dissipation of the impact energy. In case of tough interlayers, such as the well known polyvinylacetals or high strength silicones, large displacements of the interlayer occur during the impact process, contributing strongly to the total absorbed energy. The impacting body is then retained by the tough interlayer, even when a complete fracture of all glass plates has occurred.
Organopolysiloxane compositions used in the manufacture of laminated structures are known. These materials are either performed sheets made by calendering gum-type elastomers or liquid compositions. For example, Clark and Smith in U.S. Pat. No. 4,985,525, issued Jan. 15, 1991, and in U.S. Pat. No. 5,059,484, issued Oct. 22, 1991, disclose compositions having cohesive bonding to a variety of organic polymer substrates and comprising a polydiorganosiloxane gum having silicon-bonded unsaturated groups, an organohydrogenpolysiloxane, a platinum catalyst, a second polydiorganosiloxane gum containing a specified concentration of silanol groups, and a treated silica reinforcement filler. One type of treated silica disclosed by Clark and Smith is the one described by Lutz in U.S. Pat. No. 4,344,800, issued Aug. 17, 1982. Such materials are of high viscosity and cannot be applied by casting-in-place. They are also characterized by high tensile and tear strength.
Liquid polysiloxane compositions that can be cast-in-place and cure at room or slightly elevated temperature are, for instance, those disclosed by Burrin et al in U.S. Pat. No. 3,616,839, issued Jun. 30, 1967, who disclose a silicone resin cast-in-place interlayer for laminating glass with a stretched acrylic plastic sheet. One type of such silicone resin disclosed by Burrin et al uses as a cure mechanism the addition of an SiH linkage of one organopolysiloxane molecule across the double bond of an olefinically-unsaturated radical attached to another organopolysiloxane molecule. The stated advantage of such a structure is that it permits the combination of an external abrasion resistant glass surface with a plastic sheet which can function as a load bearing member. Similar compositions are disclosed in other references. For example, G.B. 2 080 378A discloses the use of inter alia such platinum-catalyzed compositions as a thermal barrier in the channels of windows. The compositions may contain a filler which is non-reinforcing for silicone rubbers. Suzuki in U.S. Pat. No. 4,477,626, issued Oct. 16, 1984, discloses platinum-catalyzed gel-forming compositions obtained by mixing a polydiorganosiloxane having at least two vinyl groups per molecule, a polydiorganosiloxane having at least two silicon-bonded hydrogen atoms per molecule, a platinum catalyst, fine silica powder and a polyorganosiloxane having at least 0.5% of hydroxyl groups The presence of the latter component is stated to produce the desired thixotropy in the compositions. Clark et al in U.S. Pat. No. 4,978,696, issued Dec. 18, 1990, disclose liquid organosiloxane compositions that cure by a hydrosilylation reaction and contain from 0.1 to about 2 weight percent of a low molecular weight polymethylvinylsiloxane. The inventive feature of Clark et al ('696) resides in the use of the low molecular weight polymethylvinylsiloxane, which lowers the modulus and increases the elongation of the cured composition sufficiently to achieve a high level of flexibility. The stated advantage of such composition is that it allows for the absorption of stresses resulting from the unequal rates of expansion or contraction of two dissimilar substrates that are bonded using the composition. Clark et al ('696) specifically require the use of an optically clear reinforcing filler. Organosiloxane copolymers, such as the ones disclosed by Daudt and Tyler in U.S. Pat. No. 2,676,182, issued Apr. 20, 1954, are specified as one class of reinforcing fillers. A second class of optically transparent reinforcing fillers includes finely divided silica of the type described by Lutz ('800).
Several properties are required in an interlayer for use in the fabrication of fire resistant laminated glass structures. They must, for most applications, be substantially transparent when cured, exhibit low shrinkage and be able to pass the prescribed fire endurance test. In case of laminated safety glass structures made from materials that themselves do not provide sufficient impact resistance to pass the test requirement, such as regular float glass (not tempered, heat strengthened, or wire reinforced), the interlayer has to be strong and tough (high tensile and tear resistance) to prevent the impacting body from penetrating the laminate structure. The interlayer should have good (ideally unprimed) adhesion to glass and its adhesion as well as physical properties should not degrade under environmental influences. A further, preferred, property is that the interlayer composition should be flowable and thus capable of convenient introduction into the space between the glass sheets. The flowable cast-in-place interlayer should rapidly cure, preferably at room temperature, to allow handling of the laminated glass structure within a few hours of casting the interlayer.
Prior art interlayer compositions have not been able to fulfill these requirements. For example, organic resin interlayers melt at high temperatures thus allowing the laminate to shear. They also have poor adhesion to glass at high temperature, thereby allowing the broken pieces of glass to fall away and permit access of the fire to the outer pane. In addition, the organic layer can be flammable. Silicone based interlayers generally have better fire resistance and less tendency to high temperature softening than the organic resins. However, those silicone interlayers that are sufficiently tough to resist the penetration of an impacting body are generally available as pre-formed sheets and, if formulated as a liquid composition, do not lend themselves to application by cast-in-place techniques, due to their high viscosity. Silicone interlayer formulations, on the other hand, that are of sufficiently low viscosity to be applied by cast-in-place techniques, lack the strength to resist the penetration of an impacting body.